This invention relates to coextruded laminate structures in which one of the layers is fluoropolymer.
U.S. Pat. No. 5,500,257 discloses the preparation of fluoropolymer composite tubing useful as automotive fuel line or hose, by first extruding a tubing of the fluoropolymer, then surface activating the outer surface of the fluoropolymer tubing by exposure to corona discharge or plasma, followed by extruding an outer layer of thermoplastic polymer onto the tubing. The fluoropolymer inner layer provides excellent chemical resistance and impermeability to fluid such as fuel when passed through the tubing. The outer layer which is preferably polyamide provides strength and abrasion resistance to the overall composite tubing. Surface activation of the outer surface of the tubing of fluoropolymer causes the extruded layer of polyamide to adhere to the inner layer of fluoropolymer to give the tubing integrity. While this tubing is useful, it has the disadvantage of requiring two extrusion operations and an intervening surface treatment, and the fluoropolymer tubing has to have sufficient wall thickness, i.e. at least 8 mils (0.2 mm), to be self-supporting until the outer layer of polyamide is applied. This wall thickness is generally thicker than necessary for providing the chemical resistance and impermeability required for the composite tubing.
Example 14 of PCT Publication WO 96/03448, published Feb. 8, 1996, (equivalent to U.S. Pat. No. 5,576,106) discloses coextrusion of polyamide, an adhesive interlayer, and fluoropolymer through a slit die to produce a laminate in one step. The polyamide and fluoropolymer layers of the laminate are each 0.9 mm (35 mils) thick and the interlayer tying these two layers together is 0.2 mm (8 mils) thick. This Example also discloses that by using a cylindrical coextrusion die instead of a slit die, tubular structures can be formed, useful in such applications as fuel hose and tubing in heat exchangers. The adhesive interlayer is a fluoropolymer which has an ethylenically unsaturated compound grafted thereon providing polar functionality to the fluoropolymer of the adhesive interlayer. The fluoropolymer adhesive starts out as a powder and the grafting reaction is carried out only on the surface of the powder. Nevertheless, the polar functionality giving the fluoropolymer its adhesive property survives coextrusion to form the adhesive interlayer tying the fluoropolymer and polyamide layers together. The teaching in this patent publication can prepare automotive fuel line or hose which can be used in place of the product of the ""257 patent, but with the advantage of using only one extrusion step. It has the disadvantage, however, of requiring the expense of the adhesive interlayer and an additional extruder for the interlayer.
The present invention provides a laminate comprising specified fluoropolymer and polyamide layers, which laminate can be formed in a single extrusion step, i.e., by coextrusion, wherein the fluoropolymer layer and the polyamide layer adhere to one another without the presence of an adhesive tie layer.
Thus, the present invention provides a coextruded laminate comprising a layer of fluoropolymer and a layer of polyamide directly adhered to one another, said fluoropolymer being a copolymer of tetrafluoroethylene with perfluorinated comonomer or a copolymer of ethylene with perhalogenated monomer, said polyamide having been adhesively activated to form said coextruded laminate to provide the adhesion between said layer of fluoropolymer and said layer of polyamide.
The laminate can be further characterized by the fluoropolymer layer being a surface layer of the laminate, particularly when the fluoropolymer of this layer is also adhesively activated, i.e. the fluoropolymer layer forms at least one exterior surface of the laminate. When the laminate is in the form of tubing, the adhesively activated fluoropolymer layer can form the interior surface or the exterior surface of the tubing.
By xe2x80x9cdirectly adheredxe2x80x9d is meant that there is no intervening tie layer present. It is the adhesive activation of at least one of the polymers forming the layers that causes the adhesion of the layers, one to the other, thereby providing integrity to the laminate without the use of a tie layer.
In one embodiment, the adhesive activation is obtained by incorporating a polymeric additive into the polyamide resin which forms the polyamide layer, the additive having polar functionality and being dispersed as fine particles within the polyamide resin. The polar functionality of the polymeric additive compatibilizes the polymeric additive with respect to the polyamide matrix, whereby the polyamide layer does not lose its strength and/or flexibility. Surprisingly, the additive dispersion in the polyamide matrix constituting the polyamide layer of the coextruded laminate also adheres the polyamide layer to the fluoropolymer layer.
In another embodiment, the fluoropolymer is also adhesively activated by polar-group functionalizing of the fluoropolymer which forms the fluoropolymer layer.
The adhesive activation of the polyamide and fluoropolymer is preferably carried out prior to coextrusion, i.e., prior to the polyamide and the fluoropolymer being fed to their respective extruders.
The laminate of the present invention is made by the process of coextrusion of the polymers described above. This process is another aspect of the invention. The polymers are coextruded as layers directly in contact with one another, so that the resultant adhesion between layers occurs without a tie layer being present, i.e., the coextrusion is carried out in the absence of any tie layer. When the fluoropolymer is adhesively activated, it is extruded as a surface layer for the laminate. In addition to the savings resulting from the absence of any tie layer, when the coextruded laminate is in the form of tubing, the thickness of the fluoropolymer layer can be reduced from the conventional minimum of 8 mils (0.2 mm) to only the thickness needed for chemical resistance and impermeability in particular applications such as fuel hose, e.g. to be less than 8 mils, such as no greater than 7 mils (0.18 mm) in thickness. Surprisingly, the polar functionalizing of the fluoropolymer, when that embodiment is used, does not adversely affect the permeability of the layer to such fluids as gasoline.